At first, as an information recording system, an example of a conventional optical recording system will be described with reference to FIG. 2. FIG. 2 is a block diagram of a conventional optical write/read apparatus. A beam emitted from a laser beam source 25 (wavelength: about 660 nm for the DVD-RAM), which is part of a head 2, is passed through a collimating lens 24 to obtain an almost collimated laser beam 22. The laser beam 22 is then applied on an object optical disk 11 through an objective lens 23 to form a beam spot 21 thereon. After that, the laser beam 22 is passed through a beam splitter 28 and a hologram element 29, etc. to be led to a servo detector 26 and a signal detector 27 respectively. The signals output from those detectors are subjected to an addition/subtraction process to generate servo signals such as tracking error signals, focus error signals, etc. that are inputted to a servo circuit. The servo circuit controls the positions of driving means 31 of the objective lens 23 and the optical head 2 entirely according to the obtained tracking and focus error signals, thereby positioning the laser beam 21 in the target write/read region. The signals added up in the detector 27 are inputted to a signal reproduction block 41. The inputted signals are sent to a signal processing circuit in which those signals are subjected to processes of filtering, equalization in frequency domain, and digitizing. The digitized signals obtained in the signal processing circuit are then processed in an address detection circuit and a decoding circuit respectively. And, according to each address signal detected in the address detection circuit, a microprocessor computes the position of the laser beam spot 21 on the optical disk 11 and controls an automatic position controlling means to position both of the optical head 2 and the laser beam spot 21 in the target recording unit region (sector).
If recording is instructed from a host computer to the optical write/read apparatus at that time, the microprocessor receives data from the host computer, then stores the recording data in its memory and controls the automatic position controlling means to position the laser beam spot 21 in the target recording region. Then, confirming that the laser beam spot 21 is positioned in the target region correctly according to an address signal received from the signal reproducing block 41, the microprocessor controls the laser driver, etc. to record the data read from the memory in the target recording region (sector).
As an example of such reproduction of information recorded at high track density, there is a system that uses three beams to cancel a cross-talk as shown in FIG. 3. Unlike the above conventional optical recording system, this system divides a laser beam almost collimated by the collimating lens 24 into three laser beams with use of a diffraction grating 30 and each of the divided laser beams is focused on the optical disk 11 through an objective lens 23. Those three laser beams come to form beam spots 43 in three tracks at an equal distance from each another on a diagonal line of each track so as to avoid mutual interference between the adjacent spots; one beam spot 43 is formed in one of the three adjacent tracks.
Such systems that use three laser beams respectively are disclosed in the official gazettes of JP-A No. 320200/09, JP-A No. 266382/2001, JP-A No. 176052/07, etc. In addition to those systems that use three laser beams respectively, there are some methods proposed for similar objects. One of the methods is to provide a cross-talk detection (study) region on the object recording medium and tracing the region with three laser beams, thereby obtaining desired information (the official gazette of JP-A No. 196840/2003, for example) and another of the methods is to form an algorithm using a relationship between a main track and its two side tracks to obtain desired information (official gazette of JP-A No. 113595/2000, for example), and still another method is to remove leak signals with use of various types of algorithms of a signal processing system (official gazette of JP-A No. 325196/05).    [Patent document 1] JP-A No. 320200/09    [Patent document 2] JP-A No. 266382/2001    [Patent document 3] JP-A No. 176052/07    [Patent document 4] JP-A No. 196840/2003    [Patent document 5] JP-A No. 113595/2000    [Patent document 6] JP-A No. 325196/05